The present invention relates to a method of separating and purifying a spent solvent discharged from a solvent extraction process in a nuclear fuel cycle, such as in a reprocessing plant for spent nuclear fuel or a nuclear fuel manufacturing plant.
The present invention can preferably be utilized in regeneration and disposal processes for such a spent solvent as described above.
A solvent prepared by diluting a phosphate, such as tributyl phosphate (TBP), with a higher hydrocarbon, such as n-dodecane (hereinafter referred to simply as "dodecane") and kerosene, is widely used in a solvent extraction step of a reprocessing process for spent nuclear fuel or of a wet scrap recovery process in a nuclear fuel manufacturing plant.
The spent solvent generated in the solvent extraction step contains deterioration products, such as dibutyl phosphate (DBP), formed as a result of degradation of a portion of TBP by an acid, heat, radioactive rays, etc. Such deterioration products adversely affect the extraction when the spent solvent is recycled for reuse. Therefore, the deterioration products are removed by alkali washing with an aqueous solution of sodium hydroxide or sodium carbonate. A radioactive waste containing the deterioration products thus removed, such as DBP, is converted into a vitrified solid or a bituminized solid by mixing the same with a vitrification additive or a bituminization additive. However, in order to stabilize a large amount of the sodium component incorporated by the alkali washing, it is necessary to use a large amount of these additives. Consequently, the development of a method of separating and purifying a spent solvent which enables deterioration products, such as DBP, to be removed without using sodium has been desired in the art.
On the other hand, methods such as vacuum freeze-drying and low-temperature vacuum distillation wherein the boiling point difference is utilized have been used as a method of separating TBP, DBP and dodecane from a spent solvent. However, they are disadvantageous in that the treatment capacity is small. Consequently, the development of a separation method having a large treatment capacity for a spent solvent has been desired in the art.
Moreover, when a spent solvent is heated to conduct distillation into components, there occur problems involving the danger of fire and also the danger that volatile nuclides undergo evaporation and sublimation upon heating, thus causing environmental contamination.